After a growing plant exhausts the nutrient resources stored in its seed, it begins to drawn in nutrients from the surrounding soil using its root system. Rapidly growing plants have a high need for nutrients. If a plant cannot access the necessary nutrients then its growth becomes limited. Such nutrient limitation can impact the overall growth of the plant, the production of seeds, such as corn kernels, and the economic return to the farmer. Corn plants, in particular, require nitrogen at least until reaching the point when tassels appear, which may be at heights of 2 m (6 feet), or more. Farmers use a range of strategies for increasing the availability of nutrients for a growing crop, most notably the addition of chemical fertilizers, for example nitrogen and phosphorus.
Generally, externally-added nitrogen has the potential to be lost from farm fields more readily than does externally-added phosphorus. Nitrate, a commonly found form of nitrogen that is negatively charged, dissolves readily in water and is lost as water runs off fields into drainage ditches or streams, or as water seeps downward into groundwater. Agricultural runoff containing significant concentrations of chemical fertilizers, such as nitrogen, can lead to degraded water quality in downstream water bodies. In addition, elevated levels of nitrate in groundwater can be a human health threat.
Ammonium is a positively charged ion that generally will bind to soil particles and will, therefore, be resistant to loss via runoff. However, in alkaline conditions, ammonium transforms into its gaseous form, ammonia, which can be readily lost to the atmosphere. Furthermore, ammonium can be transformed into nitrate—and subsequently lost from the field—via a microbial process known as nitrification.
Fertilizer containing urea is susceptible to significant loss when applied to the soil surface. Specifically, the urea is hydrolyzed, or broken down, releasing ammonia gas. However, if this happens within the soil profile, there is less chance the ammonia gas will be lost to the atmosphere; with favorable soil chemistry, ammonia is converted to ammonium, a more stable form of nitrogen. Fertilizer additives are currently marketed to reduce temporarily the rate of urea hydrolysis.
Nitrogen can also be lost through a process known as denitrification, whereby nitrate is converted to gaseous forms of nitrogen, including dinitrogen—the form of nitrogen found in the atmosphere—and nitrous oxide. Nitrous oxide carries with it several serious environmental concerns; namely, it is a greenhouse gas many times more potent than carbon dioxide, it contributes to stratospheric ozone depletion, and it contributes to smog.
Nitrogen can also be lost from the soil through microbial-mediated processes that create other gaseous forms of nitrogen. Warmer soil temperatures cause microbial processes to occur more rapidly, meaning that nitrogen fertilizer remaining in or on warmer soils is increasingly susceptible to this type of loss.
Phosphorus is most commonly found in soils as phosphate. By contrast to nitrogen, phosphorus readily binds to soil particles. Nevertheless, phosphorus can be lost from fields through soil erosion or, less commonly, via runoff if the soil can no longer bind additional phosphate because all available binding sites are filled.
Fertilizer costs, which are closely tied with the cost of fossil fuels, are significant in the production of commodity crops like corn. Fertilizer that is lost from the farm field represents inefficiency in agricultural production systems, as well as a potential loss in profit realized by the farmer. Particularly in the case of nitrogen fertilizer, the longer an externally-applied fertilizer remains on an agricultural field, the more opportunities there are for the fertilizer to be lost as described above.
Pre-season applications of fertilizer is common, either in the late fall following harvest or around the time of planting in the spring. Both fall and spring applied nitrogen has the potential of being lost from the field during heavy spring rains, plus fall applied nitrogen has several additional months on the field when it can be lost due to the various processes outlined above.
As a crop becomes established, it effectively pumps water from the soil to the atmosphere through a process known as transpiration. As a crop's leaf area increases, its ability to pump water from soil to atmosphere increases. In part, because of a crop's increased ability to pump water via transpiration, there is a reduced chance that heavier rains will lead to runoff. Nevertheless, heavy rains that lead to flooding still increase the likelihood of nitrogen loss via denitrification, especially if soils are warmer.
The substantial cost of fertilizer in the production of commodity crops like corn incentivizes farmers to adjust applications to match the needs of what their crop will ultimately require throughout the growing season. Yet, farmers are prone to over-apply nitrogen out of anxiety that there will be insufficient nitrogen available when it is required by their growing crop. Furthermore, some farmers forego in season application of nitrogen because of their anxiety about being able to get the necessary equipment on the field within the appropriate time window.
Additionally, farmers contend with a range of tradeoffs when considering the timing and size of fertilizer applications. For example, fertilizer is often cheaper in the fall, although there is increased likelihood of nitrogen losses with fall application.
Farm fields are heterogeneous, with one location potentially varying year-to-year in its nutrient status and differing from locations in its immediate vicinity. It is standard for farmers to assess soil nutrient status with periodic samples analyzed in a laboratory. Soil tests are used to estimate nutrient needs prior to the growing season, in season, or prior to an in season application of nitrogen. Independent crop consultants are commonly retained by farmers to help interpret lab analyses of soil tests and management practices. Similarly, land grant universities have extension agronomists who are able to assist farmers in these types of management decisions.
The potential for heterogeneity of nutrient status across a given field has led some to develop a soil sampling system that blends together a large number of samples taken as the equipment travels across a field. This approach may, however, mask finer-scale heterogeneity that could be used to guide variable applications of fertilizer across a field.
In recent years, instruments that measure optical properties of the growing plants are being used to indicate zones of nutrient deficiency that can subsequently be addressed with the precision application of fertilizer containing the necessary nutrient. In some cases, these instruments are used at the same time a farmer is fertilizing a field, with near-instantaneous adjustments made to meter the applied fertilizer. Strategies have been developed for mapping field zones to aid in the application of fertilizer.
The use of tractor-drawn and self-propelled equipment to manage row crops is well known. In situations where taller crops require management, the use of tractor-drawn equipment is possible to a point, beyond which, high-clearance vehicles are required. In situations where high clearance is required, it is possible to use airplanes to apply agricultural chemicals and even to seed cover crops, although airplane application is not feasible or ideal in many situations.
Corn plants, in particular, require nitrogen at least until reaching the point when tassels appear, which may be at heights of six feet or more. Conventional tractor-drawn implements are incapable of applying fertilizer when corn is so tall, which has led to the use of self-propelled sprayer systems, often referred to as “high boy” systems. Such high-boy systems are capable of straddling corn that is about six feet tall.
A typical nitrogen fertilizer used in such applications is known as UAN (liquid mixture of urea and ammonium nitrate in water). Best practices include working fertilizer such as UAN into the soil between rows of corn rather than spraying it on the soil surface. Justifications include research that indicates there will be less loss of nitrogen through volatilization and absorption by decaying plant material on the soil surface which tends to bind the UAN, inhibiting the movement of UAN downward through the soil toward the crop's roots.
The leaves of growing corn plants, in particular, can develop visible color changes if contacted by concentrated nitrogen fertilizer, such as UAN. While research suggests that there is no long term impact on corn yields, such apparent crop damage is viewed negatively by many farmers. A modification that helps to alleviate this concern with high-boy sprayers is to attach tubes to the sprayer nozzles that extend to the soil surface. Nevertheless, these dangling tubes, attached to a fast-moving vehicle, can still result in concentrated nitrogen fertilizer splashing on the corn leaves.
Because of the concern that valuable fertilizer can be lost to the atmosphere through denitrification, further modifications of high-boy systems include implements that drop down from an elevated toolbar and work the liquid fertilizer into the soil surface with a disc or coulter. High-boy systems can be used to apply nitrogen in this manner when corn plants are tall, but these systems are currently limited to corn that is about six feet tall. Furthermore, except in the case of when a coulter system is used, such equipment is not designed to apply UAN directionally at base of the plants, especially for taller corn. Rather, UAN is sprayed or integrated more or less indiscriminately between rows. However, in an effort to avoid splashing UAN directly on to the corn plants themselves, there are after-market products designed to guide the liquid stream to the ground.
Cover crops, which are generally seeded between the time that cash crops are grown, can provide a number of benefits in agriculture. A field with a cover crop may have less soil erosion. Some cover crops, which fix nitrogen from the atmosphere, can augment the amount of soil nitrogen in a field and reduce the need for applied fertilizer. As cover crops grow, they take up and store nutrients, essentially preventing them from being lost from the field in runoff or in other ways. In addition, some cover crops with deep roots can substantially reduce soil compaction.
In a crop like corn, an ideal time to seed a cover crop is when the plants are tall and their leaves are beginning to senesce (i.e., turn brown), thereby allowing sufficient light for cover crop growth to penetrate the leaf canopy. At these times, cover crops have traditionally been seeded by airplane or in some situations by customized high-clearance systems.
More recently, there has been an interest in the use of small robotic vehicles on farms. The notion of a tractor that could navigate autonomously first appeared in patent literature in the 1980s. For example, U.S. Pat. No. 4,482,960, entitled “Robotic Tractors,” discloses a microcomputer based method and apparatus for automatically guiding tractors and other farm machinery for the purpose of automatic crop planting, tending and harvesting.
In 2006, one study concluded that the relatively high cost of navigation systems and the relatively small payloads possible with small autonomous vehicles would make it extremely difficult to be cost effective with more conventional agricultural methods. Accordingly, many of the autonomous vehicles that have been developed are relatively large in size. For example, the Autonomous Tractor Corporation has touted the development of the SPIRIT autonomous tractor, which is a 102 inch wide “driverless,” tracked vehicle, theoretically capable of tilling, harvesting and hauling. The SPIRIT tractor, scheduled to be on the market in 2013, will use Laser Induced Plasma Spectroscopy (LIPS) to navigate on the field—a local system (not requiring satellites) that must be trained so that it can “learn” the layout of a particular field. The SPIRIT tractor will use RADAR to avoid unexpected obstacles, like humans or other animals.
Another example is the BONIROB vehicle, which is a 1.2 m (4 ft) wide four-wheeled robotic vehicle marketed by the German company Amazone. Yet another example is U.S. Pat. No. 7,765,780, entitled “Agricultural Robot System and Method,” which discloses an agricultural robot system with a robotic arm for use in harvesting of agricultural crops. However, none of these robot systems or vehicles is sufficiently narrow to allow for travel between typical planted rows in an agricultural field.
Despite the difficulty in maintaining cost effectiveness, a limited number of smaller agricultural robots have also been developed. For example, the Maruyama Mfg. Co has developed a small autonomous vehicle for spraying greenhouse crops. This machine is capable of navigating between rows of crops; however it is limited to operating in the constrained situations of a greenhouse. Moreover, it is not suited for the uneven terrain typical of an agricultural field.
Another example is U.S. Pat. No. 4,612,996, entitled “Robotic Agricultural System with Tractor Supported on Tracks,” discloses a tractor which traverses between planted rows on a track system. However, use of this system first requires the installation of an elaborate and potentially expensive track system within the agricultural field. Moreover, it is unclear how such a small tractor can provide coverage to a large agricultural field, much less multiple large agricultural fields, within a reasonable window of time.
Accordingly, what is needed in the industry is a device which can autonomously navigate between the planted rows and beneath the canopy of mature plants on the uneven terrain of an agricultural field to accomplish in-season management tasks, such as selectively applying fertilizer, thereby enabling the application of fertilizer throughout the life of the crop to minimize fertilizer loss in an effort to maximize the profit realized by the farmer. Moreover, what is needed by the industry is a system in which several small autonomous devices can work cooperatively together, in an efficient manner, to complete in-season management tasks within multiple large agricultural fields in a reasonable window of time, for example over the course of a day or several days to ensure that fertilizer is applied to crops at substantially the same point in their growth cycle.